I got curious by the issue and did a bit of poking around :).

'man xdm' sent me to read /etc/X11/xdm/xdm-config
which has an entry
DisplayManager.servers:         /etc/X11/xdm/Xservers
[ well it is supposed to keep this thing alive all the time :) ]

This file in turn has an entry
:0 local /usr/X11R6/bin/X

well so I go ahead and do a

'man X' and look what I found


       xdm (the X Display Manager)
               If  you want to always have X running on your dis-
               play, your site administrator can set your machine
               up to use the X Display Manager xdm.  This program
               is typically started by the system  at  boot  time
               and  takes  care of keeping the server running and
               getting users logged in.  If you are running  xdm,
               you  will see a window on the screen welcoming you
               to the system and asking  for  your  username  and
               password.   Simply  type them in as you would at a
               normal terminal, pressing  the  Return  key  after
               each.   If you make a mistake, xdm will display an
               error message and ask you to try again.  After you
           -   have  successfully  logged  in,  xdm will start up
The good   |   your X environment.  By default, if  you  have  an
stuff      |   executable  file  named  .xsession  in  your  home
    ------>|   directory, xdm will treat  it  as  a  program  (or
is here    |   shell  script)  to  run  to  start up your initial
           |   clients (such as  terminal  emulators,  clocks,  a
           -   window  manager, user settings for things like the
               background, the speed of the pointer, etc.).  Your
               site administrator can provide details.

Well by this time half the readers must be ready to press the delete button - 
but wait the /etc/X11/xdm/xdm-config file has a lot of interesting stuff. Like 
DisplayManager*resources:       /etc/X11/xdm/Xresources

so I go looking into it and I see this interesting entry
#ifdef COLOR
xlogin*greetColor: CadetBlue
xlogin*failColor: red
*Foreground: black
*Background: #fffff0
#else
xlogin*Foreground: black
xlogin*Background: white
#endif

I guess that solves your problem of black and white login screen because this
variable is set during bootup.

the next interesting entry in /etc/X11/xdm/xdm-config is
DisplayManager*session:         /etc/X11/xdm/Xsession

Well you have to read it to see what it does - to sum it up it checks whether
$HOME/.xsession is executable or not - if true then kicks it off else
it tries $HOME/.Xclients if even that doesn't exist then it tries 
/etc/X11/xinit/Xclients. God forbid if even that doesn't exist it leaves 
everything in the hand of 'xsm'.

Well then I take a peek at /etc/X11/xinit/Xclients and atthe very bottom of it
we have 
-------------------------------------------------------------------------
# Argh! Nothing good is isntalled. Fall back to fvwm2 (win95-style) or twm
/usr/X11R6/bin/RunWM --Fvwm95 || {
    # gosh, neither fvwm95 nor fvwm2 is available;
    # fall back to failsafe settings
    xclock -geometry 100x100-5+5 &
    xterm -geometry 80x50-50+150 &
    if [ -f /usr/bin/netscape -a -f /usr/doc/HTML/index.html ]; then
        netscape /usr/doc/HTML/index.html &
    fi
    if [ -f /usr/X11R6/bin/fvwm ]; then
        exec fvwm
    else
        exec twm
    fi
---------------------------------------------------------------------------
In other words if you got a really lean & mean X windows installation with no
window manager on this planet available it will give you a xterm.

Something which I noticed is that we do 'exec MyFavouriteWindowManager' without
any terminating "&" I believe if we kill the window manager it will reset the
X server and send you back to the login prompt. [I could be wrong -don't 
intend to find out right now in the middle of the night !!!]

Well I am assuming you are not using RedHat because I shudder to think what you
could have done to cause so many scripts to fail. In case you are doing
everything from scratch just take a peek at somebody's RedHat installation and
I am sure it will let you sleep in peace.

[Hmm noticed right now - you are running Debian - I don't know what it would
do but something simillar should be in place]

By the way if you survived upto this point I would recommend you read the 
complete manpage for xdm - whatever I have written took me two hours of reading
the manpages and absolutely no black magic on my part.



Regards
Mithun

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